Richard David is the report's lead writer and co-author with James Collins Jr., professor of paediatrics at Northwestern University. The scientists compared birth weights of three groups of women: African American, whites and Africans who had moved to America.

Most African-American women are of 70 to 75 percent African descent. "If there were such a thing as a (pre-term birth) gene, you would expect the African women to have the lowest birth weights," David said. "But the African and white women were virtually identical," with significantly higher birth weights than the African-American women, he said.

For black women, "something about growing up in America seems to be bad for your baby's birth weight," David said. Another argument against a genetic cause is that children of American black women rate higher for all the major causes of death in the child's first year. "Genetic diseases pop up at random in different (racial) populations," David noted. "But one group is taking all the hits. If this were a genetic problem it wouldn't fit that pattern."

David and Collins spoke with black women who had babies with normal weights at birth, comparing them with black women whose babies' birth weight was very low -- under three pounds. They asked the mothers if they had ever been treated unfairly because of their race when looking for a job, in an educational setting or in other situations. Those who felt discriminated against had a twofold increase in low birth weights. And for those who experienced discrimination in three "domains," the increase was nearly threefold.

In David and Collins' study of black women who gave birth in two Chicago hospitals, 16 percent said their partner was in jail during the pregnancy. "We interpreted this finding as another indicator of stress, but one caused by institutional rather than interpersonal racial discrimination," David said.